Conventional megasonic cleaning tanks employ a fluid filled tank having substrate supports therein and a source of megasonic energy, (e.g., a transducer) coupled to the fluid for directing sonic energy through the fluid to the surfaces of a substrate or wafer supported therein. During megasonic cleaning, the transducer oscillates between a positive and a negative position at a megasonic rate so as to generate positive and negative pressures within the fluid (and thereby coupling megasonic energy to the fluid. As the energy imparted to the fluid oscillates between positive and negative pressure, cavitation bubbles form in the liquid during negative pressure and collapse or shrink during positive pressure. This bubble oscillation and collapse gently cleans the surface of the wafer.
Particles cleaned from the wafer are carried upward via a laminar flow of fluid and flushed into overflow weirs coupled to the top of the cleaning tank. Thus, a supply of clean fluid is continually introduced to the cleaning tank from the bottom of the side walls thereof. Cleaning fluid distribution nozzles are positioned along the bottom of the sidewalls to supply various cleaning fluids through the same nozzles or through dedicated sets of nozzles.
Most conventional cleaning tanks position one or more transducers along the bottom of the cleaning tank. Acoustic waves from these transducers reflect from the surface of cleaning fluid back into transducers, and interference results in reduced power density in the tank and reduced cleaning efficiency. Due to the limited area of the tank's bottom, the number, size, placement and shape of the transducers, fluid inlets, etc., often can not be freely selected for optimal performance. Particularly, positioning the transducer elsewhere would allow a higher laminar flow of fluid from the fluid inlets, and would decrease cleaning/processing time.
Accordingly, a need exists for an improved sonic cleaning tank that provides high laminar fluid flow yet avoids the interference of incident and reflected waves.